The Contribution of Glucose Cycling to the Maintenance of Steady‐State Levels of Lactate by Hepatocytes During Glycolysis and Gluconeogenesis

Abstract

When hepatocytes from fasted rats were incubated with 10 mM glucose, there was a linear accumulation of lactate and pyruvate for about 80 min after which steady-state concentrations of these metabolites became established. The rate of glycolysis, determined with [6-3H]glucose, was constant over the entire incubation period and was 50% greater than that calculated from carbon balance studies. This suggests that one-third of the glycolytic products formed were recycled to glucose. To enable study of the factors associated with the generation and maintenance of the lactate steady state and to measure accurately the carbon balance, incubations were performed using supraphysiological concentrations of glucose (20-80 mh"). Under these conditions the initial rate of lactate accumulation and its concentration at steady state were shown to be dependent on the concentration of extracellular glucose.Rates of glycolysis were also measured using 40 mM [6-3Hlglucose and [U-'"C]glucose added alone, or in combination with a steady-state lactate concentration (3 mM). There was no effect on the rate of glycolysis determined with [6-3H]glucose, even when lactate was present in the medium. The difference in rates between measurements with the two isotopes reflects the apparent degree of glucose recycling which in the absence and presence of added lactate increased from 0.26 to 0.54 pmol C, equivalents . min-' . g-' respectively. Identical studies employing [U-'"C]lactate showed that glucose and CO, were the major products of lactate metabolism under steady-state conditions and that the formation of lactate from [1J-'4C]glucose exactly balanced the rate of lactate removal as a result of oxidation and gluconeogenesis. These studies provide evidence for the concomitant operation of glycolysis and gluconeogenesis, even in the presence of high glucose concentrations. They also demonstrate that lactate steady states are achieved not by the cessation of glycolysis but rather by the removal of lactate and pyruvate at a rate equal tlo that of their production.Keywords. Glucose cycling ; glycolysis ; gluconeogenesis ; hepatocyte ; lactate ; steady state.The liver is the principle organ responsible for controlling lactate levels in the blood [I -31. Any rise in blood lactate concentration, due for example to muscular activity, is rapidly corrected by the removal of lactate by the liver and its conversion to glucose that can then pass back to the muscle, providing the fuel for further glycolysis, a process known as the Cori cycle [4]. However, this cycling of glucose and lactate need not involve muscle but can take place entirely within the liver, with hepatic glycolysis anid gluconeogenesis occurring concomitantly [5-81. The phenomenon of glucose cycling has aroused considerable interest, bet the factors that determine the rate of carbon flow under various physiological conditions remain uncertain.A feature of the iinterrelationship between glucose and lactate metabolism reported in preliminary studies [9, 101 is that, when glycolysis is init...

Identification of a mammalian glycerol-3-phosphate phosphatase: Role in metabolism and signaling in pancreatic β-cells and hepatocytes

“…For gluconeogenesis determination, after infection with adeno-or lentiviral constructs, hepatocytes were starved in DMEM without glucose for 2 h, followed by incubation in glucose-free DMEM (pH 7.4) without phenol red, supplemented with 10 mM glycerol or 20 mM sodium lactate plus 2 mM sodium pyruvate, 2 mM L-glutamine, and 15 mM Hepes for 2 h. After incubation, the medium was processed for glucose measurement. Glycolysis was assessed by lactate content and lactate release measurement, as lactate is considered a reliable index of steady-state glycolysis in isolated hepatocytes in culture (41).…”

The Influence of Thyroid State on Hepatic Glycolysis

“…The energy requirements of observed changes in glycogen synthesis are small and ATP generation through glycolysis should be sufficient to account for the results. The conditions were such as to prohibit any substantial degree of protein or lipid synthesis although we have observed a significant degree of cycling of lactate and pyruvate back to glucose [46] after a 30-min incubation period, enough to account for an 0,-consumption of up to 0.6 pmol . min-' .…”

“…The results obtained with ['"C]glucose indicate that twice as much pyruvate was metabolised by hyperthyroid cells as by hypothyroid hepatocytes, even though the levels of pyruvate measured at 30 min were similar. The metabolism of pyruvate under these conditions reflects both reconversion back to glucose via oxaloacetate [46] and oxidation to acetyl-coenzyme A.…”

Futile Cycles Revisited: A Markov Chain Model of Simultaneous Glycolysis and Gluconeogenesis

“…14 C] lactate in the medium (Phillips et al, 1995). The metabolic fate of glycerol under these conditions was determined by the infusion of 0.14 M [U-…”